Synthesis of nanopowder can be conducted within a closed vessel using various processes. U.S. Pat. No. 6,777,639 (“the '639 patent”) describes a repetitive pulsed power nanopowder synthesis technique. In this process, a high-magnitude current pulse creates a high-density plasma from a quantity of precursor material that results in the production of nanoparticles held in a gas suspension. U.S. Pat. No. 5,460,701 describes a continuous arc process used to produce gas suspensions of nanopowder. A reaction vessel is needed that aids in the collection of the particles from the gas in which they are suspended.
In the above nanoparticle synthesis devices, a continuous flow of gas (a “quench gas”) serves as the cooling medium for the hot plasma from which the nanoparticles are synthesized. For a given nanoparticle production rate, the flow rate of this gas needs to cool the reaction products to a temperature below the rated temperature of components downstream of the reaction vessel such as piping, filters, and blowers. This quench gas is also used to convey suspended nanoparticles from the reaction vessel into a powder collection device such as a filter or electrostatic precipitator. The gas flow rate needs to clear the reaction zone of suspended nanoparticles, because suspended particles lingering in the production zone can serve as nucleation sites for the growth of larger, undesirable particles. It is therefore desirable to design the reaction vessel in such a way as to promote the smooth flow of suspended nanoparticles out of the reaction zone.
During operation, the pulsed-power process (as described in the '639 patent) generates a plasma that violently expands and mixes with the gases within the vessel. It is desirable to contain this rapid expansion within a relatively large reaction vessel to minimize contact between the walls of the reaction vessel and the hot expanding reaction products. Using a large reaction vessel prevents the vessel walls from physically interfering with the nanoparticle synthesis process and prevents damage to the reaction vessel walls. However, the large size of the vessel can lead to large zones of relatively slow-moving gas within the reaction vessel, especially if the diameter of the reaction vessel is significantly larger than the gas inlet piping. These stagnant zones allow suspended particles to linger within the reaction vessel, interfering directly with the synthesis process as well as forming opaque clouds that may interfere with optical instrumentation used to monitor the process. To sweep the particles out faster it is often necessary to increase the gas flow rate to well above the rate required to cool the reaction products. This potentially can interfere with the synthesis of the reaction products. Moreover, increasing the quench gas flow rate requires the added expense of using larger gas handling components (piping, blowers, collection filters, heat exchangers, etc.). Thus, it is desirable to employ a reaction vessel geometry that minimizes the gas flow rate required to remove the suspended nanopowder from the reaction vessel.